Piezoelectric crystal assembly



July 6, 1943.

w. R. KOCH 2,323,610

PIEZOELECTRIC CRYSTAL ASSEMBLY Filed June 4, 1942 (Iltomeg' Patented July 6, 1943 PIEZOELECTRIC CRYSTAL ASSEMBLY Winfield R. Koch, Haddonfield, N. J assignor to Radio Corporation of America, a corporation of Delaware Application June 1, 1942, Serial No. 445,718

Claims.

This invention relates to improvements in electrode arrangements for piezoelectric crystals and has special reference to an improved crystal assembly for use in electric wave filter systems and in oscillation generator systems.

It is well known that a section or plate cut from a piezoelectrically active material, such as tourmaline or quartz, may be utilized for the purpose of selectively coupling one electric circuit to another. When so used, such acrystal element, theoretically, should offer relatively high impedance to all frequencies other than its natural frequency of vibration. In actual practice, however, it has been found that the selectivity of a crystal section may depart considerably from the selectivity theoretically attainable. The reason for this is found in the fact that the interelectrode capacity between the electrodes offers a path between the coupled circuits, the reactance of which is substantially independent of the crystal, per se. In the usual filter-crystal assembly employing a four electrode resonator crystal, some interelectrode capacity between the input and output electrodes always exists. For some types of filters, it is desirable to keep the capacitance very low and to this end various relatively cumbersome auxiliary compensating or shielding means have previously been proposed, such for example as a bridge circuit or a panel or shield mounted perpendicular to the plane of the crystal in the space between the input and output electrodes. (As to this see Beers U. S. Patent 1,883,490.)

Accordingly, the principal object of the present invention is to provide an improved and relatively compact capacity-compensated four-electrode crystal assembly and one which dispenses with the auxiliary compensating or shielding means of the prior art.

The foregoing and related objects are achieved, in accordance with the invention, in a four-electrode crystal assembly, by extending the grounded electrodes over a greater area than the active input and output electrodes, for example in the patterns shown in the accompanying drawing, wherein:

Figure 1 is a partly diagrammatic sectional view of a piezoelectric crystal assembly including four electrodes which are designed, positioned and arranged in accordance with the principle of the invention to minimize the capacitive transfer of energy between the input. and output circuits which are associated therewith,

Figure 2 is a top plan view of the crystal assembly shown in Fig. 1, and

Figures 3 to 5, inclusive. are plan views of alternative electrode arrangements within the invention.

In the drawing, wherein like reference characters designate the same or corresponding parts in all figures, Q designates a quartz or other crystal element possessing piezoelectric properties and which is provided on its opposite major faces with four electrodes A, B. C and D, respectively, which preferably comprise adherent or integral coatings constituted of evaporated metal or discrete metallic films but which may be in the form of removable metal platess n a d in Fig. 1, the electrodes A and B comprise, respectively, the input and output electrodes of the assembly. These active electrodes A and B are mounted on opposite sides of the crystal Q adjacent to opposite ends thereof and are shown provided with suitable leads a and b, respectively. The other electrodes C and D are effectively connected together by grounded leads 0 and d. The said electrodes are each of an area substantially greater than the active electrodes A and B with which they are paired and are mounted in accordance with the invention with their marginal edges arranged in overlapping relation to p event the capacitive transfer of energy between the input and output electrodes A and B. Extending the grounded electrodes C and D over a greater area than the input and output electrodes A and B, with which they are paired, somewhat increases the effective impedance to the crystal, as the effective impedance depends upon the crystal area under (or over) the active electrodes.

Where, as shown in Figs. 1 and 2, the quartz crystal Q comprises an oblong element adapted to respond to a frequency which is a function of its thickness dimension, the uncovered, or uncoated spaces between the electrodes Aand D, and C and B, preferably extend across the crystal faces in a direction normal to its long axis. On the other hand, in the case of certain co'ntourmode crystals where it is preferable t"6'a'.pply a mounting force along a nodal axis of the rystal, as indicated by the spaced dots e, e in gs. 3. 4 and 5, the uncovered portion of each electrode face may run in a generally diagonal direction, as shown in Figs. 3 and 4, or lengthwise of the crystal, as shown in Fig. 5.

Other modifications will suggest themselves to those skilled in the art to which this invention appertains. Accordingly, it is to be understood that the foregoing should be interpreted as illustrative and not in a limiting sense.

What is claimed is:

i. In combination, a piezoelectric crystal, an input electrode disposed on one side of said crystal, an output electrode disposed on the opposite side of said crystal and offset from said input electrode, and a second pair of electrodes having an area larger than the area of said input and output electrodes disposed on opposite sides of said crystal and effectively connected together for minimizing the capacitive transfer of energy between said input and output electrodes.

2. The invention as set forth in claim 1 wherein said piezoelectric crystal comprises an oblong thickness mode quartz element and wherein the uncovered areas of said electrode faces extend across said element in a direction substantially normal to the long axis thereof.

3. The invention as set forth in claim 1 wherein said piezoelectric crystal comprises an oblon contour mode quartz element and wherein the uncovered areas of said electrode faces extend along said crystal substantially parallel to the long axis thereof.

4. In combination, a piezoelectric crystal, an active electrode and a grounded electrode mounted adjacent each of the major faces of said crystal, said grounded electrodes being of larger area than said active electrodes and having overlapping marginal edges for minimizing the capacitive transfer of energy between said input and output electrodes.

5. Th invention as set forth in claim 4 wherein said piezoelectric crystal comprises an oblong quartz element and wherein said overlapping marginal edges of said grounded electrode extend across the long axis of said element.

WINFIE'LD R. KOCH. 

